Calling Line Identification (CLI, often referred to as “Caller ID”) for basic call scenarios is well defined in standards and has been deployed successfully all over the world for both traditional telephone lines (sometimes referred to as POTS or Time Division Multiplexing—TDM—lines) and Session Initiation Protocol (SIP) lines. For traditional telephone lines, both basic calls and featured calls support CLI seamlessly. For a hybrid network where both traditional telephone lines and SIP lines coexist, CLI support has been very problematic and not fully supported in some cases. This presents a dissatisfaction to carriers who are trying to modernize the network by gradually migrating traditional telephone lines to SIP lines. End users who have used traditional phones for years are accustomed to correct CLI displays and may be hesitant to accept a service offering with missing or incorrect CLI displays. This problem hence can become an impediment to Class 5 network transformation as well.
As a concrete example, consider three end users—A, B, and C. CLI display update is not fully supported for call transfer scenarios when the A party is at a traditional telephone endpoint (line or trunk) and the B and C parties are SIP lines. Consider the following use case. A (a traditional telephone line) calls B (a SIP line), B transfers the call to C by putting the A-B call on hold and then dialing the C party and completing the transfer when the C party is ringing. In this scenario, the C party sees B′ CLI information. But ideally the C party display should change from displaying the B party's CLI information to displaying the A party's CLI information once the transfer is completed and the B party is out of the call. Displaying the A party's CLI information would provide accurate information on who is on the other end of the line if C answers the phone. It directly affects the end user's perception of the quality of the service offering, but is not currently supported consistently or comprehensively.
There are mechanisms proposed prior to this disclosure but these mechanisms do not solve the real world problem of CLI display for featured scenarios like call transfer in a hybrid environment where both TDM endpoints and SIP endpoints coexist. One such mechanism is to use the end-to-end SIP REFER method. This method works only if the A party supports the SIP REFER method and is able to send an INVITE with REPLACES header towards C. However, traditional telephone endpoints do not understand the SIP REFER method. In addition, INVITE with REPLACES is intended to be applicable for DIALOGs initiated by the served endpoint which is not the case for the transferee. Hence this mechanism is not applicable when one or more of the endpoints is a traditional telephone. Another option is to use SIP RFC 4916 to update the displays of the endpoints with correct CLI information. However, most phones do not support RFC 4916, hence this method is not practical for real world deployment.